1. Field of the Invention
This invention relates to the synchronization of an image taking operation of two cameras and, more particularly, to a method and apparatus which find ready application in a photo-proofing system wherein one camera may be a conventional photographic film camera and the other may be an electronic camera, such as a still video camera.
2. Description of the Prior Art
In professional photography, such as in portraiture, industrial photography or other photographic opportunities having a cooperative or an inanimate subject, several photographic frames or pictures normally are taken, or "shot", developed and selected for use by the photographer or subject. In portraiture applications, it often is inconvenient for the subject to return to the photographer's studio to undertake the selection process. Likewise, it is expensive and labor intensive for the photographer to develop a large number of preselection pictures, or "proofs" and send them to the subject for selection. In some situations, notwithstanding the skill of the photographer, a subsequent portrait-shooting session must be arranged and this adds to the time, labor and expense in concluding the portraiture process.
It has been proposed heretofore to provide a photo-proofing system by which the photographer and his subject may observe a facsimile of the pictures being taken prior to the development thereof, and thereby may select the desired photographs before development and printing even begins. If necessary, additional shots may be taken during the same session if this preliminary review is unsatisfactory. In most if not all of the photo-proofing systems which have been proposed, a video camera of the type normally used to image moving (or active) scenes generates an electronic version of the images being photographed by the conventional film camera. Typically, an optical beam splitter images the subject to both the film camera and the video camera; and a field or frame of the subject which is imaged by the video camera at the time that the shutter of the film camera is triggered is preserved.
In one type of photo-proofing system proposed heretofore, one field of the imaged subject is recorded on a magnetic tape. In another, a complete frame (i.e. two fields) is recorded on magnetic tape. And in yet another, a full field of the imaged subject is recorded in digital form on a hard magnetic disk drive. In addition to being recorded, the particular field or frame also may be displayed on a conventional video monitor, such as a television receiver, for instant "proofing".
In the video cameras used in the photo-proofing systems described above, the electronic image is in the form of a conventional NTSC video signal comprised of successive fields produced by scanning an interlaced array of photoelectric elements on a line-by-line basis. Each field is comprised of alternate lines of the projected image and the fields recur at the rate of sixty fields per second. A complete replica of the original image is formed of a frame comprised of two fields, and the frames recur at the rate of thirty frames per second. In one embodiment of the aforedescribed photo-proofing system, when the shutter of the photographic film camera is triggered, the particular field being scanned by the video camera at the time of this triggering is "captured." However, since a field essentially contains only half the optical information of the subject being imaged, the capturing of a single field provides generally poor resolution in the electronic "proof". Thus, the photographer and the subject may be able to view only a poor facsimile of the "shot" taken by the film camera and, thus, might not be able to select an acceptable photograph therefrom.
To overcome this difficulty of poor resolution in the captured electronic image, some photo-proofing systems capture a complete frame when the shutter of the film camera is triggered. However, the duration of a frame is 1/30 second which is relatively long and may be significantly longer than the duration that the shutter of the film camera is opened. While this generally may have no affect upon the photographing of portraits wherein the subject generally is stationary, the use of a full frame photo-proofing system with active subjects may result in a change in the pose of the subject at the time that the video frame is captured relative to the time that the shutter of the film camera is actuated.
Another drawback associated with photo-proofing systems is recognized in U.S. Pat. No. 4,805,037 which describes the illumination of a subject to be photographed by an electronic flash unit. As pointed out therein, synchronization of the video camera, the film camera and the flash unit is critical to assure proper flash illumination for the electronic image of the subject. Hence, the cameras and flash unit are synchronized to distribute the flash illumination relatively equally over two fields of video information which are recorded as the facsimile, or proof, of the photographed image.
The typical video camera used in virtually all of the photo-proofing systems proposed heretofore do not include a shutter. A typical shutter mechanism exhibits what is referred to herein as a "shutter release delay" which is essentially an inherent mechanical time delay from the time that the shutter is released until the time that the shutter is fully opened. This shutter release delay generally differs by a relatively small amount from one camera to another of a particular model, but varies greatly from one camera made by one manufacturer to another made by a different manufacturer. For example, the shutter release delay of one camera may be on the order of about 130 milliseconds whereas the shutter release delay of another may be on the order of about 200 milliseconds. Recently, a video still camera has been introduced which includes a shutter similar to shutters used in film cameras. For example, Model MVC-2000PF is a still video camera manufactured by Sony Corporation which includes a shutter to image a scene onto the image pickup therein. The still video camera having a shutter mechanism may be operated by a user in much the same way as a conventional film camera.
When a still video camera having a shutter is used in a photo-proofing system, synchronization of the film and video cameras becomes far more important than previously. This is because an image is not projected continuously onto the image pickup means of the video camera as it was before. Rather, an image is projected only when the shutter of the video camera is released. Therefore, to make certain that the electronic image is a precise facsimile of the latent image projected onto the photographic film requires the opening of the shutters of both cameras at about the same time. Any substantial difference in the actual release of the respective shutters may result in slightly different images being recorded by the respective cameras.
The problem of synchronizing the shutter releases of two cameras is further complicated by the need to synchronize the operation of the video camera shutter with the operation of the storage medium in which the electronic image signals produced by the video camera are recorded. For example, if the storage medium comprises a rotatable magnetic disk, as is used in the aforementioned still video camera manufactured by Sony Corporation, the image projected onto the photosensitive pickup of the camera is not "captured" until the disk rotates to a reference position. This delay in the capturing of the image is, of course, variable and is dependent upon the instantaneous position of the disk at the time that the user attempts to release the shutter. Typically, this variable delay is in the range of about 0-16.7 msec. In any event, this variable delay must be taken into account when synchronizing the release of the shutters of the film and video cameras, which typically exhibit different shutter release delays.